The present invention relates to new and useful improvements in the dry milling of sorghum grain and other cereal grains. More particularly, the invention relates to decortication and degermination of grains for the purpose of deriving therefrom coarse particles of refined endosperm, commonly known as grits.
It will be understood that cereal dry milling, within the context of this disclosure, is the practice of operating upon a cereal grain in such a manner as to fractionate it essentially in acccordance with its three major component tissues for the primary purpose of producing therefrom useful products of high starch content but low lipid and fiber contents. Said component tissues are commonly known as seadcoat, germ and endosperm. The seedcoat tissue is of high fiber content and also has a significant proportion of lipids. The germ contains the major portion of the lipids of the whole grain, and it also has the highest protein content of all tissues. The endosperm is comprised largely of starch, and has a moderate proportion of protein and only minor amount of lipids. The objectives of the milling process is therefore to produce maximum yields of refined endosperm particles with minimal amounts of seedcoat and germ tissues in adherence to or in admixture with said endosperm. The grain, throughout the milling process, is usually held in the dry state. This description is known to be unspecific as to the moisture content of the grain and only to imply that no free or unsorbed water is present. Thus, the grain may be processed at its inherent or equilibrium moisture level; or a preliminary tempering or conditioning process may be applied to increase the moisture content of the grain and to establish therein a preferred distribution of the added water among the cereal tissues for the purpose of assisting in the detachment of the tissues.
Commercial dry milling of sorghum grain is conducted generally by two known processes. One process comprises the succession of individual operations described as follows: (1) decortication, or removal of seedcoat tissues, by pearling, polishing or various other forms of abrasion or impact; (2) removal of loosened seedcoat tissue by aspiration; (3) degermination, or cleavage of the pearled grain so as to fracture the endosperm and detach the germ therefrom, thus producing a mixture of endosperm and germ particles; and (4) resolution of said mixture by a combination of processes of sifting, aspiration and gravity separation to remove the germ and accumulated fine particles and thereby produce refined particles of endosperm as the prime product. Prior to this sequence of operations, the sorghum grain may be tempered or conditioned to a limited extent to assist in removal of seedcoat tissue. The second known process is an adaptation of the common roller-mill process for milling of wheat to produce refined flour products. The sorghum grain is conditioned to higher moisture levels and for relatively longer periods of time to effect loosening of seedcoat tissue and to soften the germ, and is then reduced by an elaborate progression of roller-mill, aspiration, sifting and purifying operations. The basis principles in regard to separation of the several cereal tissues in this gradual reduction process are that endosperm will be ground to relatively small granular particles which will pass through the finer mesh screens of sifters, whereas the germ will be flattened with only moderate extent of comminution and will pass over said finer screens, and the light and flaky seedcoat tissue will pass over coarser screens in admixture with partially reduced particles of the more dense tissues and be lifted therefrom by aspirators or purifiers. The refined endosperm is thus recovered in the form of small particles such as meals and flour, in contrast to the larger particles, or grits, obtained from the milling process first described.
Degerminators for use with grain sorghum have heretofore been used experimentally. These devices have consisted of a cylindrical wire brush rotating within a perforated cylinder, the grain being fed directly into the peripheral space between the brush and cylinder. Such a device is described in the 1965 issue of "American Miller and Processor". A similar device which has been used experimentally substitutes a cylinder having ribs or bars for the wire brush. These devices have a very low capacity, and the wire brush type is very impractical as the bristles soon wear off and are likely to break and leave wire particles in the product mix. Furthermore, none of the prior devices distribute the grain uniformly in the operating space, with the result that production rate is limited and no means is provided to assure uniform treatment of the individual grains, with undesirable effects such that the subsequent separation steps are rendered more complicated and expensive. In none of these prior devices is the grain fed into the interior of a hollow rotor and spread and uniformly distributed therein before it is discharged into a peripheral space where decortication and degermination is to take place.
Patents with which applicants are familiar are as follows: U.S. Pat. Nos. 3,885,464 May 27, 1975; 3,703,200 Nov. 21, 1972; 3,734,752 May 22, 1973; 2,108,655 Feb. 15, 1938; 3,603,365 Sept. 7, 1971; 3,222,183 Dec. 7, 1965; 3,498,796 Mar. 3, 1970.
While these patents relate to broadly similar objectives, the subject matter of the claims of the present invention is not disclosed or suggested therein. In U.S. Pat. No. 3,885,464 the grain is inserted at one end of a cylinder and removed from the other end after being acted on by spaced disc-like polishing stones. In U.S. Pat. No. 3,703,200 the grain is fed to a gap between the end face of a rotor and the adjacent end of the casing. The grain is not fed to the interior of the rotor to be discharged from rotor openings. U.S. Pat. Nos. 3,734,752; 2,108,655; and 3,603,365 all have conical degerminating rolls and there is no hollow rotor into which grain is fed to be discharged from peripheral openings into a milling space. U.S. Pat. No. 3,222,183 discloses no apparatus and merely suggests the use of conventional scourers, hullers, decorticating apparatus or brushes. U.S. Pat. No. 3,498,796 relates to a heat and moisture treatment prior to being acted on by a conventional roller mill.